1. Technical Field
The subject invention relates to a method of non-contact detection and monitoring of elastic stresses in a semiconductor wafer.
2. Description of the Prior Art
Semiconductor wafers are subjected to internal and external stresses during growth and processing. Internal stresses are generated by crystal defects, such as dislocations, precipitates, and point defect agglomerates. External stresses are created after the wafer undergoes thin-film deposition and thermal annealing. The thin-film deposition adds a film layer of material to the wafer. Then as the wafer undergoes thermal processing, if the film and the wafer have different rates of thermal expansion, the external stress may be formed. These stresses cause premature and uncontrollable breakage of the wafers and the failure of manufactured devices.
In the U.S. Pat. No. 5,972,782, issued Oct. 26, 1999 to the inventor named herein, there is disclosed a method of ultrasound treatment (UST) of hydrogenerated poly-Si film on transistors. The ultrasound enhanced hydrogenation is disclosed therein as UST stimulated release of atomic hydrogen from the electrically nonactive state (H-reservoir) followed by subsequent hydrogen trapping at dangling Si bonds in grain boundary regions and surface interfaces. As disclosed in this prior patent, sandwiching a wafer with a transducer adjacent an acoustic wave detector carries out the method.
The subject invention provides an enhanced method of inducing, detecting and monitoring of elastic stresses in a semiconductor wafer by allowing the wafer to freely respond to the induced vibrations.
The invention presents a method of detecting and monitoring elastic strains in a semiconductor wafer, said method comprising the steps of coupling the wafer to a transducer having a periphery, operating the transducer to produce ultrasonic vibrations at a predetermined wave length, and propagating a standing wave through the wafer in response to the ultrasonic vibrations. The method is characterized by extending the wafer in a cantilevered section from the periphery of the transducer to a distal end, and measuring the amplitude of the standing wave in the cantilevered section.
As the wafer extends in a cantilevered section from the periphery of the transducer, the cantilevered section is free to vibrate unimpeded by the transducer thereby to produce a more accurate evaluation of the residual strains in transistor wafers.